Lauren Shuler-Donner has mentioned the possibility of an X-Men 4, but I hope there isn't one. I'm a HUGE X-Men fan, and I had not been disappointed with the films, until X-Men 3 was released.
Unfortunately, the mistakes and f*#& ups in that movie were too solid to undo. They completely destroyed the Phoenix storyline and ruined the potential for a kick ass Rogue/Cyclops. By killing off Xavier, they managed to solidify the death of a good X-Men 4...without the Professor to hold the stories/movies together, there can't be another, at least that's my opinion.
Besides, I would hate to see the replacement for any potential actors that might not want to return. To add to the mess, there's no more Cyclops or Jean!
In a perfect world, Fox and the rest of the world would deem X-Men 3 the bastard child in the films and completely redo it. Same cast and all. Would it be too much to ask to continue from where X2 PERFECTLY left off?
I will forever be left with the desire to see PHOENIX kick (destroy) some ass on screen. X-Men 3 could've been Famke's movie.